This invention relates to an improved directional coupler for use in a CATV system and, in particular, to a directional coupler having a relatively low tap value and a relatively high return loss and directivity.
In CATV systems, wide band television signals are typically transmitted by coaxial cable over relatively long distances. Amplifiers are situated at various intervals along the lines to maintain the signal strength at or above a desired operating level. Samples of the broadcast signal are extracted from the line at increasing distances from the amplifier. The broadcast signals are tapped from the main transmission line by means of directional couplers.
A directional coupler in common usage includes a first current sensing transformer that is connected in series with the center conductor of the cable to apply a first current sample to an output tap point. A second transformer is placed in shunt between the center conductor and the grounded outer conductor of the cable and is arranged to apply a second voltage sample to the output tap. Cablecast signals moving over the line from the amplifier toward the load cause the sample signals to be additive at the output tap point. Reflected signals moving in the opposite direction along the cable on the other hand cause the sample signals at the tap point to be 180.degree. out of phase whereby the signals cancel each other. The coupler in effect prevents these unwanted reflected signals from reaching the subscriber's receiver where it would produce distortions.
Rheinfelder in U.S. Pat. No. 3,543,163 describes a typical cable arrangement in which an amplifier is used to transmit a wide band signal over a specified length of cable. Seven directional couplers are situated at intervals of about 150 feet. As noted by Rheinfelder, insertion losses and line loss are experienced as the signal moves further away from the amplifier. As a result, the tap value of the couplers is reduced periodically in order to maintain the signal level at the subscriber's set at a level required for good reception. The nominal tap loss of the coupler becomes compressed the further the coupler is located from the amplifier. Normally, the tap value of the coupler is generally not allowed to drop below 12 dB because the reflection losses at these low values are typically high.
Some added disadvantages of conventional directional taps having low values are poor frequency response, high insertion losses and reduced isolation.